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Traffic along Central Square was moderate Friday evening, and many people were walking through the popular area on their way home from work or to one of its many restaurants. Cambridge police Officer Raymond Pina was on foot patrol, standing in front of a CVS and observing the comings and goings.

It was 7:24, still daylight, when several hundred bicyclists participating in the once-a-month Critical Mass ride approached Prospect Street in the westbound lane of Massachusetts Avenue. The event was part of a regular series of group rides, staged by cyclists in cities across the nation, intended to highlight how unfriendly cities are to cycling.

The Cambridge cyclists were or derly, Pina later noted in a police report, but when they arrived at the intersection, several cyclists stopped and held up traffic, allowing other cyclists to pass through a red light. Drivers were blaring their horns and yelling at the cyclists to move, and the cyclists were yelling back at the drivers, Pina wrote.

That’s when things started to go wrong. Pina said he saw the potential for a vehicle to hit a cyclist. He walked into the street and started ordering the cyclists to stop, but they just swerved around him, he said.

Yonatan Kurland, a 28-year-old bike rider, wound up on the ground, and the cause is now the subject of much discussion in the world of cycling.

Pina stated in his report that Kurland was traveling swiftly, weaving through other cyclists on the street, and would have clipped him if he hadn’t grabbed the handle bars on Kurland’s bike. The officer said the momentum dragged him.

Kurland’s account differs drastically from Pina’s. Kurland said he was traveling slowly in the bicycle lane, about 30 feet from the intersection, when Pina approached, grabbed the front of his bike, and pushed him to the ground, leaving Kurland with minor road rash.

Police issued Kurland a $20 ticket for operating to endanger.

Dan Riviello, a spokesman for the Cambridge Police Department, said, “We believe Officer Pina acted within his lawful authority in responding to the defendant’s refusal to comply with his lawful commands to stop.’’ He said Pina has not been reprimanded or penalized for his handling of the incident.

Kurland said he will appeal the ticket and plans to file a complaint against Pina today. “I just don’t understand it, for him to give me a ticket for endangering. I wasn’t endangering anyone. I think he used an inappropriate amount of force for an unwarranted stop.’’

Kurland said he joined the ride because “it sounded like a lot of fun.’’ He went to Copley Square Friday evening and met up with about 200 cyclists of various experience levels. The pace of the ride was leisurely, about 5 miles per hour, and unencumbered because they didn’t wait for red lights. They traveled through Chinatown, on the Rose Fitzgerald Kennedy Greenway, and in the Theatre District before heading to Cambridge on Massachusetts Avenue.

Critical Mass rides began in 1992 in San Francisco. Now, cyclists in more than 300 cities worldwide participate in the monthly rides. The events, which often tie up traffic, are viewed as a form of social protest, but riders say it is more a celebration of cycling. In most cities, including Cambridge, bicyclists have the same rights to the road as motorized vehicles. Ride participants acknowledge that they use a tactic known as “corking’’ to seal intersections from traffic, allowing cyclists to pass.

Joel Pomerantz was one of about 50 people who participated in the first Critical Mass ride in San Francisco and coined the corking term. “A bunch of us just proposed the idea of riding our bikes home together from work,’’ he said in a telephone interview yesterday.

“Back then, we had no idea what shape it would take, but it grew by 80 percent every month and after a year it was in the thousands.’’ He said people visiting the city from other states and countries participated, and took the experience with them, starting Critical Mass rides elsewhere.

“When you experience being in the street without dealing with the danger of cars, it’s magical, and I think that feeling is what led to it spreading throughout the world,’’ said Pomerantz, 50. He said people have their own reasons for participating, but one purpose that seems to resonate among cyclists is to showcase an alternative to automobiles.

But it appears there have been a growing number of incidents in which Critical Mass participants have been ticketed, arrested, or had their bikes confiscated, Pomerantz said.

In New York City in 2008, a police officer knocked a cyclist off his bike and onto the sidewalk during a Critical Mass ride on Broadway. That officer, who had been on the force for 11 days, resigned and was convicted this past April of making a false statement, after stating in his report and telling superiors that the cyclist initiated the contact.

Participants acknowledge that there are occasional arguments between cyclists and drivers, but say for the most part, the events are fun-filled and peaceful.

Boston police say they have not had any incidents with Critical Mass riders. The department’s bicycle officers monitor the events. Elaine Driscoll, police spokeswoman, said, “we always encourage bicycle riders as well as operators of motor vehicles to follow the rules of traffic.’’